1. Field of the Invention
This invention relates to the design and fabrication of a magnetic tunnel junction (MTJ) MRAM array, particularly to a design for locking (creating a stable magnetization state) un-selected array devices and unlocking (creating a less stable magnetization state) selected array devices.
2. Description of the Related Art
The magnetic tunnel junction (MTJ) basically comprises two electrodes, which are layers of ferromagnetic material, separated by a tunnel barrier layer, which is a thin layer of insulating material. The tunnel barrier layer must be sufficiently thin so that there is a probability for charge carriers (typically electrons) to cross the layer by means of quantum mechanical tunneling. The tunneling probability is spin dependent, however, depending on the availability of tunneling states with different electron spin orientations. Thus, the overall tunneling current will depend on the number of spin-up vs. spin-down electrons, which in turn depends on the orientation of the electron spin relative to the magnetization direction of the ferromagnetic layers. Thus, if these magnetization directions are varied for a given applied voltage, the tunneling current will also vary as a function of the relative directions. As a result of the behavior of an MTJ, sensing the change of tunneling current for a fixed potential can enable a determination of the relative magnetization directions of the two ferromagnetic layers that comprise it. Equivalently, the resistance of the MTJ can be measured, since different relative magnetization directions will produce different resistances.
The use of an MTJ as an information storage device requires that the magnetization of at least one of its ferromagnetic layers can be varied relative to the other and also that changes in the relative directions can be sensed by means of variations in the tunneling current or, equivalently, the junction resistance. In its simplest form as a two state memory storage device, the MTJ need only be capable of having its magnetizations put into parallel (low resistance) or antiparallel (high resistance) configurations (writing data) and that these two configurations can be sensed by tunneling current variations or resistance variations (reading data). In practice, the free ferromagnetic layer can be modeled as having a magnetization which is free to rotate but which energetically prefers to align in either direction along its easy axis (the direction of magnetic crystalline anisotropy). The magnetization of the fixed layer may be thought of as being permanently aligned in its easy axis direction. When the free layer is anti-aligned with the fixed layer, the junction will have its maximum resistance, when the free layer is aligned with the fixed layer, the minimum resistance is present. In typical MRAM circuitry, the MTJ devices are located at the intersection of current carrying lines called word lines and bit lines. When both lines are activated, the device is written upon, ie, the magnetization direction of its free layer is changed. When only one line is activated, the resistance of the device can be sensed, so the device is effectively read. Such an MTJ device is provided by Gallagher et al. (U.S. Pat. No. 5,650,958), who teach the formation of an MTJ device with a pinned ferromagnetic layer whose magnetization is in the plane of the layer but not free to rotate, together with a free magnetic layer whose magnetization is free to rotate relative to that of the pinned layer, wherein the two layers are separated by an insulating tunnel barrier layer.
In order for the MTJ MRAM device to be competitive with other forms of DRAM, it is necessary that the MTJ be made very small, typically of sub-micron dimension. Parkin et al. (U.S. Pat. No. 6,166,948) teaches the formation of an MTJ MRAM cell in which the free layer is formed of two antiparallel magnetized layers separated by a spacer layer chosen to prevent exchange coupling but to allow direct dipole coupling between the layers. The free layer thereby has closed flux loops and the two layers switch their magnetizations simultaneously during switching operations. Parkin notes that sub-micron dimensions are needed to be competitive with DRAM memories in the range of 10–100 Mbit capacities. Parkin also notes that such small sizes are associated with significant problems, particularly super-paramagnetism, which is the spontaneous thermal fluctuation of magnetization in samples of ferromagnetic material too small to have sufficient magnetic anisotropy (a measure of the ability of a sample to maintain a given magnetization direction). To overcome the undesirable spontaneous thermal fluctuations in MRAM cells with very small cross-sectional areas, it is necessary to make the magnetic layers thick. Unfortunately, the size of the switching field increases with layer thickness, so the price paid for a thermally stable cell is the necessity of expending a great deal of current to change the magnetic orientation of the cell's free layer.
Some degree of anisotropy is necessary if an MTJ cell is to be capable of maintaining a magnetization direction and, thereby, to effectively store data even when write currents are zero. As cell sizes have continued to decrease, the technology has sought to provide a degree of magnetic anisotropy by forming cells in a wide variety of shapes (eg. rectangles, diamonds, ellipses, etc.), so that the lack of inherent crystalline anisotropy is countered by a shape anisotropy. Yet this form of anisotropy brings with it its own problems. A particularly troublesome shape-related problem in MTJ devices results from non-uniform and uncontrollable edge-fields produced by shape anisotropy (a property of non-circular samples). As the cell size decreases, these edge fields become relatively more important than the magnetization of the body of the cell and have an adverse effect on the storage and reading of data. Although such shape anisotropies, when of sufficient magnitude, reduce the disadvantageous effects of super-paramagnetism, they have the negative effect of requiring high currents to change the magnetization direction of the MTJ for the purpose of storing data.
Another way to address the problem that high currents are needed to change the magnetization direction of a free layer when the shape anisotropy is high, is to provide a mechanism for concentrating the fields produced by lower current values. This approach was taken by Durlam et al. (U.S. Pat. No. 6,211,090 B1) who teach the formation of a flux concentrator, which is a soft magnetic (NiFe) layer formed around a copper damascene current carrying line. The layer is formed around three sides of the copper line which forms the digit line at the underside of the MRAM cell.
In conventional MRAM design, both the word and bit lines that intersect at the location of a particular MRAM cell must be carrying currents in order for that selected cell to be switched and for a 0 or a 1 to be written thereon. For the large number of other cells that lie along only the current carrying bit line, or along only the current carrying word line, but not at their intersection, only the field of a single line is experienced. Such cells are called half-selected. In an MRAM array, the half-selected cells must not be switched and the selected cell must be switched.
Every cell in an array can be thought of as being potentially under the influence of the superposition of two magnetic fields. Ideally, if only one line is carrying current, the local field at the cell position should be insufficient to switch the cell and the cell can be said to be in the un-switched zone of the local magnetic field. A problem arises, however, because variations in cell formation, particularly when cells are extremely small, can allow a cell to switch even when it is in the un-switched zone of the local magnetic field.
The purpose of the present invention is to create a design margin for each cell, so that individual variations in cell structure would be insufficient to place a non-selected cell within the switched zone of its local magnetic field. Such a design margin can be achieved if the magnetization of a cell can be locked into a highly stable “C” state after each act of writing on the cell (holding it in a stable state during write operations on other, nearby, cells), and if the cell can then be placed into a less stable “S” state when it is actually being written upon. In the method of the present invention, the capability of a free layer to be placed into C or S states is provided by magnetostatically coupling it to a soft magnetic layer (a cladding layer) formed around a bit line and by building in a small amount of magnetic anisotropy into the free layer. This additional magnetostatic interaction, along with the conventional magnetic fields produced by currents in the word and bit line, produces two states of flux closure within the free layer, which are the C and S states that are desired.
Cladding layers surrounding current carrying write lines have been taught by others. Bloomquist et al. (U.S. Pat. No. 6,661,688 B2) teaches a write line structure in which a cladding layer nearly completely surrounds a write line below a memory storage device. The cladding layer has an open space above the write line, so that it effectively forms two poles immediately adjacent to the storage device. The structure is said to provide a greater field at the storage device for a given current in the write line.
Bhattacharayya et al. teaches an array of magnetic memory cells which are written upon by segmented write lines. The line segments are each cladded with high permeability, soft magnetic material to increase their magnetic fields for given currents.
Sharma et al. (U.S. Pat. No. 6,593,608 B1) uses a cladded bit line to serve as a seed layer for the formation of a soft magnetic reference layer (ie a pinned layer) within a double reference layer magnetic memory cell.
Jones et al. (U.S. Pat. No. 6,555,858 B1) discloses a self aligned clad bit line structure formed within a trench.
Rizzo (U.S. Pat. No. 6,430,085 B1) teaches a method of forming a cladding layer of magnetic material so that the layer has a shape anisotropy parallel to the conducting line that it clads and also has an induced anisotropy that is not parallel to the shape anisotropy. The combination of the two anisotropies enhances the permeability of the layer, thereby increasing the magnetic field for a given current.
Although all of the foregoing cited prior art teaches the use of a magnetic cladding layer for the purpose of enhancing the magnetic fields used for switching an MRAM cell, none of the prior art teaches the use of such cladding to magnetostatically couple to a free layer so as to create states of greater and lesser stability.